Barclays was mired in fresh controversy on Wednesday night after handing almost £9m to a top banker who left following the Libor scandal and after one of its highest profile non-executive directors suddenly quit, taking the toll at the top to four.

Jerry del Missier, who resigned after telling subordinates to reduce the bank's Libor submission during the October 2008 banking crisis, was reported to have been handed £8.75m cash as part of his leaving package.

Alison Carnwath, who also chairs the property company Land Securities, cited "personal reasons" and time pressure for her immediate resignation from the board of Barclays, which is still reeling from the exit of chief executive Bob Diamond.

The bank's chairman Marcus Agius, who is also in the process of leaving but is due to present the bank's half-year profits on Friday, will now face questions about the pay off for the Canadian-born banker Del Missier, who had just been promoted to chief operating officer.

Del Missier was a close lieutenant of Diamond's and appeared before the Treasury select committee to explain why he had instructed the Libor submission to be lowered.

Del Missier said Diamond had told him to cut the rate after a conversation with Paul Tucker, the Bank of England deputy governor, something both Diamond and Tucker insist was not the case.

When reports of a disagreement over Del Missier's leaving bonus emerged, the bank had insisted it was keen to foster debate in the boardroom.

Shadow Treasury minister Chris Leslie called on del Missier to follow Diamond and waive the bulk of his payoff. "Having resigned from Barclays over the Libor fixing scandal, people will find the scale of this award completely inappropriate. Bob Diamond rightly waived most of his pay off and Mr del Missier ought to do the same," Leslie said.

Barclays declined to comment.

There was speculation of boardroom tensions even before the Libor scandal broke, between Carnwath and Agius during negotiations over Diamond's bonus in the spring.

Former investment banker Carnwath wanted Diamond to waive a £2.7m bonus – part of a £17m pay deal – to show leadership but she eventually agreed to the payout. At the bank's annual meeting in April the payout sparked a shareholder revolt over the bank's pay policies.

Carnwath herself also faced a shareholder rebellion, with 22.5% of investors failing tosupport her because of her status as chairman of the remuneration committee. However, when it later emerged that she had tried to stop the bonus, some investors made it clear they were keen for her to stay. One said: "She seemed to have been the one board member asking the right questions."

However, others pointed out that she has eight other jobs or advisory roles in addition to Barclays. They include a non-executive directorship at the troubled hedge fund group Man Group, where she faced another shareholder revolt about her continued membership of the board. She has been on the board of Man for 11 years - which contravenes governance guidelines. Some 33% of investors failed to support her election to the board of Man. At Land Securities her election to the board was not so controversial.

Carnwath has not yet commented on the recent events at Barclays, but has spoken to accountancy magazine economia for a cover interview which is to be published on Friday. In the interview she explains that the Barclays remuneration committee faced "a lot of pressures" when dealing with the top echelons of the bank. She adds: "It's not easy, and we don't always get it right."

"I sometimes wake up worried – or don't go to sleep at all because I'm anxious," she said. "That's not so good. Last night I lay awake fretting about something. I then have to stand up and somehow all the worry drains out of my feet, or I write things down."

In a statement announcing her departure, Carnwath blamed the pressures of too many jobs.

"With regret I have concluded that I am no longer able to devote sufficient time to my role as a director of Barclays given my other commitments. I would like to thank my colleagues on the board for their support and I wish Barclays continuing success in the future."

Agius, who will leave once his successor is found, thanked her for her contribution since joining the board in August 2010.

The board has now handed a central role to former top accountant Sir Michael Rake, who becomes deputy chairman – although he has ruled himself out of the chairman's job despite expressing interest when Agius first quit.